Gas reacting method

ABSTRACT

Gas reacting apparatus and method are described for wet mass transfer of solute gases from a gas stream with a liquid or slurry reacting medium capable of chemisorption of solute gases in the gas stream. The apparatus comprises an elongated conduit means and plurality of dual-fluid spray means coaxially spaced in series within the conduit means and countercurrently or cocurrently directed to the gas stream for spraying the liquid or slurry reacting medium into the conduit means to form a plurality of spray contact zones or uniformly-distributed fine droplets in individual gas-liquid contact zones wherein intimate contact of high interfacial surface area between the sprayed liquid or slurry and the gas stream is effected to remove solute gases from the gas stream. The gas-liquid contact zones are separated by demisters which agglomerater and remove liquid droplets from the gas stream before it passes to the next zone. A demister also is provided at the outlet.

REFERENCE TO RELATED APPLICATION

This application is a continuation-in-part of copending U.S. patentapplication Ser. No. 438,047 filed Nov. 30, 1989, which itself is adivision of U.S. patent application Ser. No. 243,720 filed Sep. 13, 1988(now U.S. Pat. No. 4,963,329) which itself is a continuation-in-part ofU.S. patent application Ser. No. 020,953 filed Mar. 2, 1987 (now U.S.Pat. No. 4,865,817).

FIELD OF INVENTION

This invention relates to gas/liquid, gas/liquid/solid andgas/gas/liquid mass transfer, and more particularly to amechanically-aided gas reacting apparatus and method for masstransferring solute gases from an industrial or utility gas stream intoa liquid or slurry reacting medium, if necessary in conjunction withsimultaneous particulate matter removal, wherein the mass transfer maybe a purely physical phenomenon or may involve solution of the solutegas in a liquid or a slurry suspension, followed by chemical reactionwith one or more constituents in the liquid or slurry reacting medium.This invention also relates to a gas reacting apparatus for effectinggas-gas reactions in fluid flow communication with an integrated wetseparation of the resultant reaction products which are in the form of afinely divided particulate matter. While not limited thereto, theinvention is particularly well suited for the removal of SO₂ and othergaseous pollutants from waste gas streams such as those emanating fromelectrical utilities, smelters and industrial boilers.

BACKGROUND OF THE INVENTION

For maximum efficiency, it is desirable to make a gas reacting apparatuswherein a high interfacial surface area coupled with turbulent mixingand long residence time are effected simultaneously and which, as such,is capable of removing both solute gases and particulate matter, eitherseparately or simultaneously with high efficiency, with the former beingseparated via a gas-liquid, gas-liquid-solid or gas-gas-liquid reaction,depending on the application. To-date, as far as we are aware, there iscurrently no one type of gas reacting apparatus available that iscapable of achieving high performance for all of the above criteria, dueto a compromise generally being made between generation of very finedroplets for affecting very high surface area on one hand and longresidence time on the other.

One of the methods for separating particulate matter in solid or slurryform from a gas stream wherein a dirty gas stream enters a conduit atone end and is moved through it by a fan at the other end and where afine spray of liquid, preferably water, is cocurrently discharged intosuch a gas stream upstream from the fan is described in U.S. Pat. No.4,067,703, issued Jan. 10, 1978, the disclosure of which is incorporatedherein by reference. The patent disclosure, while showing ahighly-effective method for removal of particulate matter from a gasstream, does not teach how the apparatus can be used as a gas reactingapparatus for removing solute gases. Also, in many aspects, thetechnique disclosed therein does not provide the absorbing and reactingenvironment required for effecting high removal efficiency of solutegases. For example, in the foregoing prior art patent, the mixture ofgas and particulate matter enters only a single contact spray zoneprovided by one nozzle in which an atomized liquid spray is injectedcocurrent to the dusty air stream flow. While this mode of operation asdisclosed has proven to be highly effective for removing particulatematter and effecting lower pressure drop in the apparatus whereinparticles were collected primarily by impaction upon the finely-dividedwater droplets introduced, followed by further agglomeration andimpaction on the fan blades as the gas moves through the device, theresidence time available for mass transfer is too short and theeffective interfacial surface area and turbulence generated by a singlecontact spray cocurrently oriented to the gas stream are not sufficientto effect high removal efficiencies of solute gases of relatively lowsolubility in aqueous solution. It is, therefore, desirable to providefor an improved gas reacting apparatus and method which overcome some ofthe shortcomings of the foregoing prior art apparatus in which increasedavailable residence time, interfacial surface area and turbulence aregenerated to result in accelerated absorption and reaction kinetics andintimate gas/liquid contact and thus, in high removal efficiency of bothsolute gas and particulate matter.

While high interfacial surface area, turbulent mixing and long residencetime for effecting accelerated mass transfer of solute gases andeffectively separating particulate matter are the major criteria in gasreacting apparatus selection, often a compromise must be made betweenremoval efficiency on one hand and operating reliability on the other.Several other factors then also enter into consideration, such as slurryhandling without plugging, turndown, and gas and liquid distribution.

The basic processes for removal of solute gases from gas streams,particularly flue gas desulfurization processes, are based onreadily-available, low-cost absorbents in the form of an aqueous slurry,such as a lime or limestone slurry, or a clear aqueous solution, such ascaustic or ammoniacal solutions. Various prior art methods are in use tobring the above absorbing and reacting media into intimate contact withthe pollutant-laden gas. Packed bed and perforated trays, which areknown to be efficient gas absorption and reaction devices, are usuallythe first choice for designers of flue gas desulfurization (FGD)systems, but experience has shown that they are not completelysatisfactory. Both perforated trays which bubble the gas through a thinlayer of liquid, and packed beds, which pass the gas over solid packingelements that are wetted with the liquid have many narrow passages whichare subject to plugging especially if particulate loads are heavy, or ifprecipitates are formed during the chemisorption process. Suchconditions can be minimized by careful process design, but thepossibility of scaling under upset conditions still exists andcompromises reliability. Another principal disadvantage of both of theabove types of scrubbers is their extremely limited turndown capability.

Consequently, heretofore, the gas reacting devices of preference and theones that would seem to be the answer have been the venturi or openspray tower wherein the internal complexity is low and yet where arelatively large surface area of the liquid is generated per unit volumeof gas treated. While the above devices have evolved considerably overthe last decade in a way to improve their performance and to remove someof their shortcomings, the current trend in the design particularly ofFGD systems, is away from venturis to spray towers or combinationtowers. The venturi design, although capable of producing a relativelylarge liquid surface area for contact with the gas stream, was abandonedlargely because the very short liquid/gas contact time (attributablelargely to the absorbing medium being introduced cocurrently to the gasstream in the throat of the venturi) results in low sulfur dioxideremoval. Also, being a relatively high energy device, it is incapable ofproducing an evenly distributed regime of droplets at high densityunless an `overkill` situation exists wherein excess energy in the formof velocity pressure is added to the gas stream to provide for therequired uniform distribution. Spray towers, on the other hand, have fewinternal components in the gas/liquid contact zone and the use of spraysappears to offer an easy way of increasing the surface area exposed tothe gas. However, the sprays are usually introduced at the top of thespray tower and drop by gravity in counter-current flow to the gasstream. To avoid being entrained in the gas stream, the normal size ofthe droplets sprayed is in the order of 1000 to 2500 microns indiameter. Thus, to increase the surface area exposed to the gas phaseand residence time, very high liquid to gas (L/G) ratio and large towersmust be employed, all of which substantially increase the capital andannual cost requirement. To effect good gas distribution, a large numberof spray nozzles must be used, so that the tower cross-section isuniformly covered with the spray pattern. However, failure of one or twonozzles usually creates a path of least resistance through which the gascan flow, thereby reducing the efficiency of the apparatus.

In addition, the large size of droplets used in spray towers reducessubstantially the capability of the apparatus to efficiently remove dustparticles in the low particle size range, typically less than 3 microns.With the larger droplets, the decreased gas-liquid surface area can becompensated for by increasing the tower size, the number of sprayheaders, and circulation rates of the scrubbing liquor, all of whichincrease the tower space requirement, thereby initial cost and energyconsumption. Droplet entrainment and mist elimination, while rathereffectively being addressed by the production of larger droplets, canstill be the "Achilles heel" of spray tower operation, because it is theonly part of the operation where gas flow must be somewhat restricted.These limitations and the fact that the spray and venturi apparatus eachoffers advantages not shared by both, have given rise to the developmentof combination gas reacting devices. These combination arrangementsgenerally combine the features of venturi and spray apparatus into onemodule. These recent designs offer greater performance, allowing highremoval efficiency of both gaseous pollutants such as SO₂ andparticulate matter such as fly ash, but at a very high cost. It is,therefore, desirable to provide an improved gas reacting apparatus whichcombines all of the advantages offered by venturis and spray towers intoone apparatus.

SUMMARY OF THE INVENTION

The problems and disadvantages associated with prior art systems areovercome by the present invention by providing a gas-reacting apparatusand a method which is simple, economical and capable not only ofproviding good turndown and gas-liquid distribution, but also capable,on the one hand, of generating high turbulence and many fine droplets ofan aggregate surface area many times larger than produced in a spraytower of considerably larger size and, on the other hand, of providingfor a much longer available residence time and higher surface area thanin a venturi, thereby effecting high removal efficiency of both solutegases and fine particulate matter and yet operating the apparatus withsubstantially decreased amounts of liquid, low energy and spacerequirements. It has been shown that the amount of liquid used by theimproved gas reacting apparatus is only about 2% of that required by asuitable spray tower with comparable efficiency.

According to the invention, a gas stream containing solute gases or bothsolute gases and particulate matter is passed through a conduit andcontacted while flowing through the conduit by at least two sprays ofliquid or slurry, preferably injected countercurrent to the gas stream.

In the conduit, the liquid or slurry absorbing-reacting medium is finelyatomized by nonplugging, dual-fluid nozzles, which are preferablycentrally disposed and spaced in series in the conduit to form two ormore contact spray zones, and adapted to spray droplets in the rangeabout 5 to about 100 microns, more usually about 5 to 30 microns. Byspraying such liquids or slurries into a suitable reaction chamber, atremendous number of droplets are generated along with very high surfacearea. For example, if only 5 micron droplets are generated, eachkilogram of water yields about 1.5×10¹³ droplets which have a surfacearea of about 1200 square meters. On the other hand, in a traditionalsystem, if only 1000 micron droplets are generated, each kilogram ofwater yields about 1.9×10⁶ droplets which have a surface area of about 6square meters. These surface area figures as shown above are by an orderof magnitude greater than generated by the commercially-availabledevices presently used for this service. Since the mass transfer that agiven dispersion can produce is often proportional to (1/D), finedroplets are greatly favoured.

Upon intimate contact of the solute gas and particulate matter with thesprayed absorbing-reacting medium, transfer of the solute gas andparticulate matter from the gas stream to the absorbing-reacting mediumtakes place. The removal of the solute gases so effected may be a purelyphysical phenomenon or may involve solution of the solute gas in aliquid or a slurry suspension, followed by chemical reaction with one ormore constituents in the liquid or the slurry medium, to form a solubleby-product or a solid reaction by-product. The resultant liquid orslurry-laden gas stream may be subsequently drawn into a slowly-turningfan that provides turbulent mixing and additional residence time plus anenvironment for continued absorption and reaction, and for efficientcoalescence or agglomeration of the entrained, sprayed liquid or slurryand its subsequent removal from the system by a simple gravity drain inthe fan casing. An entrainment separator is located downstream from thefan to complete the removal of agglomerated liquid phase (includingslurries) from the system. Where little or no particulate material ispresent in the gas stream, the fan may be replaced by a much moreefficient I.D. fan downstream of the conduit to draw the gas streamthrough a suitable static demister.

The apparatus of the invention may also include means for quenching andcooling a hot gas stream, such as that emanating from electric utilitiesor smelters, with an aqueous solution (water, or other liquids) prior tothe removal of the solute gases.

The apparatus of the invention may also include an effluent hold tankfor closed loop recycling of the absorbing-reacting medium and itsregeneration with fresh make up feed, plus a pumping means to introducethe absorbing-reacting medium into the spray nozzles at the appropriatepressure.

In addition, demisters may be provided between the spray devices in theconduit, with the collected liquid passing countercurrent to the flow ofgases through the conduit.

Overall, what has been developed is an improved gas-reacting apparatusin which accelerated absorption and reaction of solute gases in anabsorbing/reacting medium can be effected due to the large surface area,intimate contact, relatively long residence time, and turbulent mixingprevailing therein, thereby overcoming the problem of the prior art, asdiscussed above.

While the invention will be described further, particularly withreference to the removal of solute gases, either by absorption orabsorption accompanied by chemical reactions, it is to be understoodthat the invention is also useful in the conduct of gas-gas reactionsand subsequent wet separation of the resulting reaction product, in theremoval of particulate matter, in the humidification of gases and inreaching a thermal equilibrium between a gas and a liquid.

In a preferred embodiment, the absorption, with or without accompaniedreactions, is conducted in the improved gas reacting apparatus whereinthe unexhausted reacting medium and the reaction products areagglomerated and thereby removed from the gas reacting apparatus as acoherent liquid or slurry mass, depending on the reacting systemselected. In most instances the resulting slurry can be recirculateduntil some optimal concentration is reached, at which point a bleedstream can be removed for further treatment to recover product or forregeneration and recycling purposes, while fresh makeup feed isintroduced into the system prior to recirculation.

In another embodiment, a gas stream containing solute gases and a liquidreacting medium for the gas are introduced into dual fluid mixingnozzles of the type described herein having a pair of inlets, one foreach incoming stream, and a common outlet. The confluence of the twostreams in the nozzle creates turbulence which causes the two streams tointimately mix and react substantially instantaneously with each otherthereby to produce reaction products which are in the form of a gassolute or finely divided solids. Wet separation of the above resultantreaction products is subsequently carried out as taught by the abovepreferred case.

One important feature of the improved gas-reacting apparatus resides inits ability to remove both solute gases and particulate mattersimultaneously with high efficiency, due to the large effectiveinterfacial surface area and the excessively large number of dropletsintroduced to the system, coupled with turbulent mixing and sufficientresidence time that can be effected therein. Still another significantadvantage of the improved gas reacting apparatus, particularly incomparison with the venturis of the prior art, is its ability toaccommodate a very high turndown ratio through a simple adjustment ofthe gas-side pressure drop across the spray nozzles or the amount ofliquid sprayed or both simultaneously. Yet another advantage is anability to provide spray zones of uniform density and, therefore, toyield even gas distribution due to the nozzles being coaxially spacedapart in series within the conduit. The spray zones completely cover thecross sectional area of the conduit and yet without overlapping oneanother, thereby providing good gas and liquid distribution even underupset conditions associated with a nozzle failure. This is preferablyachieved with a unique dual-fluid, atomizing spray nozzle design of thetype depicted in the drawings described below that has more precisegas-liquid mixture control and allows for the flexibility required tocontrol size and number of droplets necessary for efficient removal ofsolute gases. The dual fluid spray nozzles generally operate at about 20to about 100 psi, usually at about 20 to about 70 psi, preferably about25 to about 55 psi. The cumulative results of the above-describedadvantages is a gas reacting apparatus which is more economical, moreefficient, more compact and easier to handle than any other moreconventional device. Also, being a relatively small piece of equipment,it can be custom fitted/retrofitted or configured to meet variousspecific site requirements.

These and other characteristic features and advantages of the inventiondisclosed herein will become apparent and more clearly understood fromthe further description given in detail hereinafter with reference tothe attached drawings which form a part thereof.

In one preferred embodiment, a contact chamber is provided located aheadof the scrubber for the removal mainly of particulates from the incominggas stream and is useful, not only in the treatment of gas streamscontaining solute gases which contain particulates but alsoparticulate-contaminated gas streams which do not contain such solutegases.

In such chamber, the entrance and exit are located on opposite sides ofa vertically-located baffle extending normal to the gas flow. Such anarrangement causes the incoming particulate-laden gas stream to impingeon the baffle and then to pass under it to reach the exit. Theperformance of the contact chamber is significantly enhanced by theintroduction of spray nozzles of the type described above for removal ofsolute gases from the gas stream, one of such nozzles being located tospray cocurrently with the gas stream flow and the other located tospray countercurrently to the gas streams.

The nozzles usually are located in the entrance and exit respectively ofthe chamber and impinge on the baffle and preferably are arranged sothat the sprays also substantially fill the inlet and outlet ducts andthe entrance of the gas stream to and the exit of the gas stream fromthe contact chamber. The dynamic action of these fine sprays on theparticulate-laden gas stream combined with the structure of the contactchamber results in removal of significant quantities of particulate fromthe gas stream, often up to 90% or more, regardless of the particlesize.

The gas stream passing from the contact chamber is significantlydepleted with respect to particulate content, enabling very high overallefficiencies, generally in excess of 98%, of particulate removal to beeffected when the contact chamber is used as a prescrubber before theconduit.

The contact chamber also provides the additional residence time oftenrequired to achieve more than about 99% removal of certain acidic gases,notably SOx and NOx, from the gas stream via the presence of suitablereactants contained in the liquid sprayed into the contact chamber,whether particulate materials are present in the gas stream or not.

The contact chamber also may be used in a pre-scrub mode to removecertain nuisance gases prior to treatment for the removal of the targetgas. For example, when scrubbing for SO₂ removal and where the SO₂ is tobe recovered by regeneration of the scrubbing media, it is necessary toremove SO₃ gas and H₂ SO₄ fume before removal of SO₂. The removal ofthese gases can be accomplished in a contact chamber using arecirculating mode which may contain H₂ SO₄ +SO₂ (saturated) and aftersaturation with SO₂, does not remove any further SO₂ from the gasstream.

In addition, the contact chamber serves to provide effective quenchingof hot gas streams to the adiabatic dew point of the gas stream.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic sectional view of a gas reacting apparatus and itsaccessories having two separate single spray contact zones constructedin accordance with one embodiment of this invention;

FIG. 2 is an enlarged schematic sectional illustration of a preferredsingle-orifice dual-fluid nozzle (Turbotak type) used in the apparatusof FIG. 1;

FIG. 3 is a schematic view of a portion of the conduit shown in FIG. 1illustrating a cluster (multiple orifice) nozzle incorporated into onecontact spray zone and includes a sectional view taken on line C--C;

FIG. 4 is a perspective schematic view of the gas reacting apparatus ofFIG. 1;

FIG. 5 is a perspective schematic view of an alternative fan arrangementused with the apparatus of FIGS. 1 and 4 but having a common exhaustoutlet for both gas and coalesced liquid;

FIG. 6 is a schematic representation of a two-step application of thepresent invention to a typical coal or oil-fired boiler exhaust gasstream for the removal of gaseous pollutants and fly ash;

FIG. 7 is a perspective, schematic representation of an alternativeembodiment of a gas reacting apparatus of the invention illustrating adouble-loop approach to absorption of solute gases with low reactingmedium usage; and

FIG. 8 is a schematic representation of a preferred form of a contactchamber for gas quenching and particulate removal, as well as to provideexcellent contact for acidic or other gas removal;

FIG. 9 is a schematic representation of an alternative form of gascontact chamber for use in environments where space constraints do notpermit normal horizontal flow; and

FIG. 10 is a schematic illustration of an alternative form of gascontacting apparatus consisting of three contact stages and constructedin accordance with a further embodiment of the invention, for removal ofa solute gas from a gas stream, employing countercurrent flow ofscrubbing liquor and gas stream.

DESCRIPTION OF PREFERRED EMBODIMENTS

Referring to the drawings, the gas reacting apparatus 10 shown in FIG. 1comprises in combination an elongated housing 12 defining a primaryreaction zone and a fan 30 defining a post-reaction and coalescencezone.

The elongated housing 12 comprises a relatively straight conduit,preferably of circular cross-section, having an inlet 60 forintroduction of gas stream containing solute gases alone or incombination with particulate matter and an outlet 62 at the other endfor the entry of the resultant liquid or slurry-laden gas stream intothe slowly turning fan 30. The conduit 12 may be positioned in anyorientation with respect to the ground level. However, thegenerally-horizontal position or an orientation in which the conduit ispositioned at a slight angle with respect to the ground level to permitgravitational flow through the conduit, are preferred.

Within the conduit 12 are positioned a number of atomizing spray nozzlemeans 14 for discharging liquid or slurry sprays countercurrent asillustrated (or cocurrent, if desired) to the gas stream flowing throughthe duct 12. The atomizing spray nozzle means 14 provide a very finespray and are capable of delivering droplets in virtually any sizedistribution or quantity required. Typically, the range of about 5 toabout 30 microns liquid droplets is preferred. By spraying liquid orslurry in the above droplet size range into the conduit 12 a tremendousnumber of droplets having a very large interfacial mass transfer area isproduced. For example, if only 5 micron droplets are generated, eachkilogram of liquid will yield about 1.5×10¹³ droplets which have asurface area of about 120 square meters. These figures are orders ofmagnitude greater than generated by any other known contacting device.

In the preferred form as illustrated, liquid or slurry atomizing spraynozzle means 14 comprises dual fluid nozzles each capable of producingthe above droplet size distribution. However, a Turbotak dual-fluidnozzle (shown in FIG. 2) utilizing gas, i.e. air, steam, etc., to impartthe energy required to atomize a liquid is very suitable for thispurpose, particularly when using a slurry as a reacting medium. One ofthe features of the Turbotak nozzle of the type shown in FIG. 2 is thaterosion is virtually non-existent. This results because the liquid flowor the slurry flow is thought to be contained in an envelope of gas asit passes through the orifice of the device. Multiple-orifice nozzleswhich may be used in the present invention are described in U.S. Pat.No. 4,893,752, assigned to the assignee hereof and the disclosure ofwhich is incorporated herein by reference.

For maximum turbulent mixing and gas-liquid contact time, the scrubbingliquid or slurry preferably should be introduced at a sufficient nozzlepressure and velocity countercurrently to the gas stream to be scrubbedto form the desired spray pattern needed to cover substantially all ofthe cross-sectional area of the interior of the conduit 12 within areasonable distance, e.g. 5 feet from the nozzle. The geometry of thespray issuing from the nozzles 14 and the exact orientation of thenozzles 14 with respect to the conduit 12, apart from being coaxiallyspaced, are not critical. However, for a circular conduit 12, nozzles 14producing a conically-shaped spray pattern, preferably with a sprayangle flow 15° to 90°, is the most advantageous to give adequatecoverage of the conduit cross-section. To effect maximum hold-up ofliquid and gas/liquid contact time, countercurrent flow is used and theatomizing gas pressure preferably should be high enough to impart to theliquid droplets sufficient force to overcome the velocity of theincoming main gas stream, so that no reversal of the sprayed liquid bythe high velocity incoming gas stream can occur until a fully developed,conically-shaped spray pattern, with its extremity touching the conduitwall surface, is established, at which point the sprayed liquid isturned back by the incoming gas stream and becomes suspended therein. Inthis way, if all the energy expended in the sprayed liquid istransferred to the main gas with minimum loss to the conduit wall, avery high degree of turbulence results as the liquid and gas moving inopposite direction come together and the liquid is forced to reversedirection. This high degree of turbulence and increased liquid hold-upand liquid/gas contact time provides extremely efficient contact betweenliquid and gases to yield a very effective and accelerated mass transferof solute gases to the absorbing/reacting medium.

The rate of the flow and pressure of air through the nozzle and thus thedegree of atomization is controlled by a pressure reducing valve 22connected by conduits 20 to the nozzles 14. Gas pressures in the rangeof about 20 to about 60 psi, preferably about 25 to about 50, issupplied to the nozzle by a conduit connected to a gas pressure source23 through a gas regulator (pressure reducing valve) 22. Under suchatomizing pressure conditions and a liquid usage of from about 0.25 to1.0 U.S. gallon per 1000 acf (actual cubic feet) of gas treated, theTurbotak dual-fluid nozzle has been shown to be capable of generatingliquid droplet sizes in the range of about 5 to about 100 microns withthe majority of droplets having a size of about 5 to 30 microns.

For improved mass transfer operation, there may be a number of atomizingspray nozzles 14 employed within the conduit 12. The nozzles 14 arecentrally disposed, countercurrently or cocurrently oriented to the gasflow, axially-spaced apart in series in the conduit and adapted to spraydroplets primarily in the size range from about 5 to 30 microns, therebycreating a number of well back-mixed zones in the conduit 12. Suchorientation of the nozzles results in very high turbulent mixing andhigh interfacial surface area for mass transfer. While, depending on theatomizing pressure employed, the spray nozzles preferably should bepositioned and spaced apart in series in the conduit, so that theconically emerging spray patterns do not substantially overlap eachother. Generally, in the above atomizing pressure range proposed,spacing of approximately four to eight feet was found to be adequate.

It has been found that the use of separated, spaced spray nozzles 14 toprovide at least two separate gas/liquid contact spray zones in whichoppositely moving sprayed liquid and gas come together and the sprayedliquid is forced to reverse direction, provides for removal of typicallyover 99% of the sulfur dioxide and over 99.6% of the particulate matterfrom a synthetic gas stream when scrubbing with aqueous caustic solutionof 0.5M. This high efficiency is accomplished with the use of about 0.5USG per 1000 acf of gas treated which is only 10% of that required bymost scrubbers with comparable efficiency.

An important feature of the scrubber apparatus of the present inventionresides is its low energy requirement. The approximately 1 to <5 H.P.expended into the liquid per 1000 acf of gas treated and a gas pressuredrop of 0"±W.G. measured across the device are considered to be verylow. To accomplish similar removal effects by mechanically increasingthe gas flow rate by means of blowers pulling through a venturi throatinvolves greater energy coupled with inferior results.

Another significant advantage of the gas reacting apparatus of thepresent invention, particularly in comparison with systems of the priorart, is its ability to accommodate a high turn-down ratio when the flowof the gas stream is decreased because of decreased boiler load withoutthe need for adjustments by moving parts. As can be seen from FIG. 1,the turndown capability of the gas reacting apparatus is not affected bysome mechanical limitation. In the gas reacting apparatus of the presentinvention, the interfacial area is not dependent on the gas flow rate orthe pressure drop. Hence, the solute gas removal efficiency increaseswith reduction in gas flow. One method to regulate make-up feed rate isby controlling the effluent pH. Here a pH electrode probe activates asignal that regulates the position of control valve 53 to control therate of make-up feed through line 55. Other make-up feed control systemsmay be used, such as controlling the inlet gas flow and solute gasconcentration or controlling the outlet solute gas concentration as thecontrol variable.

Although the gas reacting apparatus has been described with reference tosingle orifice spray nozzles creating separate spraying zones, it isdesirable and practical in large scale applications where conduits oflarge size are used to substitute for the single spray nozzles with amultiple orifice nozzle (i.e. a cluster nozzle such as described in theaforementioned U.S. Pat. No. 4,893,752) with a combined spray patternwhich substantially covers the cross-section of the conduit in order toobtain maximum effectiveness and space utilization. A schematic view ofportion of the conduit illustrating a cluster nozzle incorporated intoone contact zone is shown in FIG. 3. In most large-scale applications,the ducts in which the nozzles are mounted are rectangular incross-section, rather than circular. (see e.g. FIG. 10).

The apparatus 10 of the present invention also comprises a low speed,motor driven fan 30 downstream from the last spray nozzle 14. The fan 30is connected to the outlet of the conduit 12. In particular, fan 30 isof the radial-blade centrifugal type and comprises a shaft 32 having abladed wheel 34 fixed thereto, the shaft and the bladed wheel beingcoaxially positioned or supported in a volute casing 36. In particular,fan wheel 34 (impeller) comprises a disc shaped member 37 fixed to theshaft 32, a plurality of blade members 38 extending from the disc 37 andequally-spaced around the shaft 32, and an annular rim 39 fixed to theedges of blades 38 and disposed in a plane parallel to the plane of thedisc 37. This type of structure, as shown in FIG. 1, was found to beself-cleaning and particularly suitable for severe duty. Other impellertypes, such as the forward curved, backward curved or inclinedstructures, may be used but are not considered to be as suitable as thesimple radial-bladed fan illustrated.

The casing of the fan 30 is formed to include an inlet 40 having aninner diameter smaller than the diameter of annular element 39. Inlet 40is connected to the open end of conduit 12, and an annular joint 41 isprovided to seal the connection. The fan opening preferably should besized to match the size of the conduit or vice versa. However, a taperedinlet 40 or a conduit gradually growing smaller toward the inlet of thefan 30 can also be employed, causing the compressible part of the gasstream to speed up, either at the hub of the fan or in the conduit whilethe incompressible part of the gas stream, i.e. "fly ash" and liquiddroplets, slows down, relative to the velocity of the gas. The increasein the relative velocity between the two phases results in lower gasphase resistance and thus better scavenging action against solute gas byliquid droplets. Also, the larger differences in velocities of theliquid droplets and the gas occur in such tapered inlet caused impactionand results in better scrubbing action against particulates. Anothermethod to improve the turbulent mixing at the inlet to the fan is by theuse of variable guide vanes to impart pre-rotation to the incoming gasstream in an opposite direction to that of the impeller rotation.

The fan drive shaft 32 is connected to the drive means in the form of anelectric motor 42 for rotating shaft 32 and fan wheel 34 at a relativelylow speed. The motor also includes suitable means 43 for maintaining thespeed in a desired range. Fan 30 also includes means in the form of angas exhaust passage from fan 30. Exhaust passage 44 is at the upperportion of casing 36 as viewed in FIGS. 1 and 4 and is connected to oneend of an output duct 46 for exhausting clean gas from the apparatus.The connection of passage 44 to duct 46 is sealed by a joint 47. Fan 30further includes means in the form of an opening 48 provided in volutecasing 36 at the lower end, as viewed in FIGS. 1 and 4, for removing orcollecting sprayed liquid and particulate matter separated from the gasstream. Opening 48 is in fluid flow communication with a relativelyshort conduit or passage 52, disposed generally vertically into a sludgetrap in the form of an open top tank 54. In slurry-based operation, thetank preferably should include an agitator means (not shown in FIG. 1)to keep the solids in suspension. The tank 54 preferably should includeabsorbing/reacting liquid 56 up to a level above the bottom of the tube52. In some cases, it may be desirable to exhaust both the clean gas andthe absorbing/reaction-liquid from the same exhaust passage 44 forsubsequent separation. Such an arrangement is shown in FIG. 5.

The distance between the last spray nozzle 14 and inlet to the fan 30 isnot critical when the nozzle is countercurrently-oriented to the maingas stream flow. However, in applications where the nozzle is directedcocurrent to the gas stream flow, a minimum distance upstream from theintake of the fan 30 generally is required to permit theconically-shaped spray to fully develop and fill completely thecross-section of the conduit 12. In general, a distance of three to fourfeet has been found to be satisfactory, depending on the gas velocity inthe duct.

The fan 30 also provides means to withdraw and move the gas stream andto overcome the pressure losses across the apparatus. In the fan, thereis provided turbulent mixing and additional residence time plusenvironment for continued absorption and effective coalescence of theliquid droplets and their removal from the system. Much turbulence canalso be effected in countercurrent operation as oppositely moving liquiddroplets suspended in an atomizing gas and the main gas stream cometogether and the liquid droplets are forced to reverse direction and topass through at least two spray contact zones formed by two or morespray nozzles.

The liquid-laden gas is drawn into the vortex of the slowly-turning fan30 in the direction indicated by arrow 62 from which the solutegas-laden droplets and the collected particulate matter exit into theliquid or sludge trap provided by opening 48 at the bottom of fan 30, asviewed in FIG. 1. Clean gas and some entrained droplets not removed bythe fan, exit through the fan exhaust opening 46. The majority of theliquid droplets first coalesced in the vortex created by the fan 30 growin size and then impinge and constantly coat the fan blades 34, to forma layer of coalesced liquid and solids that adheres to the rotary fanblades and is separated from them mostly on the edges of the blades bythe effects of centrifugal force, moving outward in so doing so as toform an attendant annular coarse spray zone to further remove solute gasand particulate matter. The liquid droplets adhering to the blades runover the blades, washing them of collected particles.

The particulate matter and solute gas-laden liquid is collected as itreaches the fan housing 36 and fan casing and drains by gravity into asump 54 through a sealed drain 50. Because the impeller 34 and the fancasing 36 are not coaxially aligned, the annular space between theimpeller and the fan casing increases toward the exhaust opening,thereby preventing any blockage and interference with the clean gasthroughput capacity by the sprayed liquid. Within the sprayed annularzones formed as a result of centrifugal force imparted on the liquidcoating the blades the main gas stream that has been agitated by theimpeller 34 comes into intimate contact with the reacting liquiddischarged from the blades 34, so that for all practical purposes,additional removal of solute gases remaining in the gas stream continuesto take place.

The flow of clean gas with some entrained liquid droplets not removed bythe fan, continues through the exhaust ducting 46 in the direction ofarrow 64, from which it can be discharged directly to the atmosphere orinto an entrainment separator for final removal of the entrained liquiddroplets. It was found that the fan, upon rotating forward, can separate85 to 90% of the liquid droplets suspended in a gas stream whilebackward rotation can separate some 95% of the suspended liquid.Therefore, for gases containing heavy dust loadings or where a higheroverall removal efficiency is required, a backward turning fan normallyis recommended, although the lower flow rate and static pressure maynecessitate either a larger fan or a booster fan in the system.

The provision of the fan, which constitutes an integral part of the gasreacting apparatus, makes it possible to use the same elements thatserve to move the gas and coalesce the liquid droplets also to providethe turbulent mixing, additional residence time plus mass transfer areasfor continued absorption of solute gases and removal of particulatematter from the gas that is to be treated.

When the above-described scrubbing system is employed solely as anabsorber, i.e. where no particulate matter is present, it is possible toeliminate the coalescing fan as an integral part of the scrubber andconduct the fine droplet laden gas stream directly to anappropriately-designed mist eliminator where the entrained liquiddroplets are separated from the gas stream. The selection of theappropriate mist eliminator is obvious to those skilled in this art. Apreferred mist eliminator consists of one or more banks of chevronblades designed for this purpose. Other devices can be employed, such asKimre filament-type preformed pads, etc.

The purpose in eliminating the fan is to reduce the size and cost ofthis equipment which also provides greater flexibility in equipmentinstallation. The pressure drop in the system is accommodated by an I.D.fan which is normally placed between the absorption equipment and thestack. Such I.D. fans have no pre-imposed limits on fan speed and so aremuch less costly and much more efficient than the style of scrubber fanused in the scrubbing system when particulate levels demand such.

In most coal fired power plants and similarly large power producingplants, extensive particulate control devices are conventional and areinstalled to remove >99% of the particulates in the gas stream. For suchinstances where SO₂ control equipment is to be retrofitted, it would bepossible to eliminate the scrubber fan as indicated above. One option toassist in further removal of remaining particulate is to install acontact chamber after the installed particulate removal equipment wherethe warm gas stream is cooled to its adiabatic dewpoint and through theuse of the nozzles, up to 90% of the particulate still in the gas streamcan be scrubbed out while not effecting removal of SO₂ (see FIGS. 8 and9). SO₂ removal then is effected in the absorption section which then isfollowed directly by a mist eliminator and again, a scrubber fan is notrequired in this instance.

In bleach plant scrubbers where the off-gases contain both chlorine(Cl₂) and chlorine dioxide (ClO₂), multiple stage absorption of theabove gases takes place with agents that remove the chlorine and reducethe chlorine dioxide and neutralize the resulting acids. Again, since noparticulate is present, it is possible to eliminate the scrubber fanwhile adding an I.D. fan to pick up the pressure drop and gas flowthrough the system.

The entrainment separator 70 shown in FIG. 4 to which the clean gas isdischarged from the fan 30 is used to separate entrained liquid notseparated by the fan 30. Typically, a Chevron-type separator which maybe followed by a special mist eliminator packing (Kimre preferred) bothcontained in the same housing were found sufficient to clean the gas ofany suspended liquid. The clean gas then is discharged to the atmospherevia duct 72 at 100% relative humidity, but virtually free of liquidwater content.

Referring again to FIGS. 1 and 4, a fresh make up of reacting mediumfeed is added to a recirculation tank 54 through line 55. From the tank54, the scrubbing material is drawn through a pump 17 and is introducedinto the atomizing spray nozzles 14. Reacting liquid recovered from theclean gas by the fan 30 and the entrainment separator 70 is returned totank 54 for reuse, while spent scrubbing liquid is discharged throughline 57.

Since gas flow in the gas reacting apparatus 10 is unrestricted,pressure drops are low, typically not exceeding 2 to 3 inches W.G.including the entrainment separator. This pressure drop is normallypicked up by the fan so that typically, the pressure drop across thesystem, flange to flange, is zero inches W.G. If desirable, the fan canalso pick up total system pressure drops up to about 6 to 8 inches W.G.

FIG. 5 illustrates an alternate fan arrangement in which a cyclonicseparator 74 is used to effect separation of gas and liquid drawn intothe fan 30.

Referring to FIG. 6, there is illustrated therein the application of thepresent invention to a typical coal- or oil-fired boiler exhaust gas forthe removal of SO₂ therefrom.

As seen, the method lends itself to the use of existing ductwork andI.D. fan, depending on the layout of these in an available plant.Constraints of residence time and temperature of a particularapplication determine whether the existing layout is practical.

As shown in FIG. 6, the gas originating in a flue gas duct 102 from aboiler 100 and exiting gas coolers 104 at temperatures normally rangingup to about 250° C., but not limited to this range, enters a scrubbingarea 106 for simultaneous SO₂ and fly ash removal using the proceduresdescribed above. Adjacent the inlet of the fan 108 a scrubbing medium isinjected countercurrently into the incoming flue gas stream throughinjection 110 to form at least two separate scrubbing zones covering thecross-sectional area of the duct 112 adjacent to the fan 108 whereby theflue gas is scrubbed. After separation of the suspended liquid from thegas by the fan 108 and a downstream entrainment separator 114 andfurther reheating by gas heater 116, a clean gas is discharged to theatmosphere through stack 118. Quenching sprays (not shown in FIG. 6)also may be incorporated where the flue gases are hot to serve to cooland saturate the gas stream with water vapour prior to scrubbing.

Referring now to FIG. 7, there is shown therein a perspective schematicview of a double-loop slurry approach to effect better utilization ofslow reacting solids in suspension such as limestone or iron oxide. Theuse of a double-loop slurry procedure offers greater flexibility becauseextreme operating conditions can be segregated into discrete areas ofthe double loop system, allowing separate chemical and physicalconditions to be maintained. In the double loop slurry procedureillustrated in FIG. 7, a low pH slurry solution contacts the enteringgas stream in an initial reacting loop 200 comprising an elongatedconduit 202 and a plurality of atomizing spray nozzles 204 centrallydisposed and spaced apart from each other in the conduit 202, andadapted to spray slurry into an incoming gas stream whereby some solutegas removal takes place. The slurry-laden gas stream exits from theconduit 202, and enters a hydrocyclone 206 via a tangential inlet 208and swirls down about the vortex finder. The swirling separated slurryconcentrate flows down the cone section to the apex opening 210 which issealed by a joint to the top of a vertically disposed conduit, the otherend of which terminates in a sludge effluent hold tank 212. Theslurry-free, partially-clean gas passes upwards through the vortexfinder to the outlet 214, then to another conduit 216 which is part of asecond reacting loop designed for almost complete removal of theremaining solute gas.

In the second loop, a high pH slurry or solution is contacted with thepartially clean gas in the conduit 216, where the bulk of the solute gasremoval takes place. The second loop comprises an elongated conduit 216,a plurality of spray nozzles 218 coaxially disposed in series to form anumber of reacting zones, a fan 220, an effluent hold tank 222 and anentrainment separator 224. Spent slurry from this loop is discharged tofirst loop via pump 226 and line 228 where the unused reagent isconsumed, thereby proving efficient reagent utilization. Fresh make-upreagent need be added only in the second reacting loop.

This type of design, incorporating two reacting loops in conjunctionwith the gas reacting apparatus of the present invention, takesadvantage of the concept of contacting a gas stream containing thehighest solute gas concentration with the lowest liquor alkalinity in afirst loop to effect good reagent utilization and relatively low solutegas removal and the highest liquor alkalinity with the lowest solute gasconcentration in a second loop to effect poor reagent utilization, butgood solute gas removal. The reduced solute gas removal in the low pHloop (lower alkalinity) is more than offset by improved performance ofthe high-pH loop (higher alkalinity).

Referring now to FIG. 8, there is illustrated therein a preferredcontact chamber 300 which effects an initial treatment of the gas streamand provides a feed to a scrubbing apparatus comprising a single spraynozzle 310 spraying liquid into the gas flowing in the duct 312 to a fan314, operating in the manner described previously. The contact chamber300 is intended to increase turbulence and residence time of the gasstream in a manner superior to the three spray nozzle arrangement ofFIG. 4. This arrangement is of particular significance when a mixture ofacid gas and particulates is to be processed with a high level ofparticulates.

The contact chamber 300 is enlarged in volume in comparison with theduct 312 and comprises a baffle 316 located transversely to the gas flowand a pair of nozzles 318, 320, each arranged to spray liquid at thebaffle 316. The contact chamber 300 is able to remove over 90% of theparticulates contained in the entering gas stream in line and theresulting slurry is conveniently drained, usually continuously, from thelower portion of the chamber 300 by line 322.

It may be necessary to agitate the liquor contained in the lower portionof the chamber to maintain particulates in suspension to facilitateremoval of the slurry, especially if large quantities of particulatesare removed from the contact chamber relative to the amount of liquidused therein.

For removal of fly ash and sulfur dioxide from a coal-fired boiler,water sprays from nozzles 318 and 320 may be used in the contactingchamber 300, which would remove substantial amounts of fly ash but onlyminor quantities of sulfur dioxide. The solids may be separated from theslurry removed by line 322 by thickening and/or filtration andthereafter sent to landfill. The aqueous phase from such separation,which is acidic from the dissolved SOs, may be recycled with make-up tothe contacting chamber nozzles or may be made basic and used as make-upliquor for the SO₂ removal stage at the nozzle 310. The nozzle 310 isfed with a basic aqueous solution to remove the gaseous SO₂ in the duct312 downstream of the contacting chamber 300.

In the removal of sulfur dioxide from a particulate-laden gas stream, itgenerally is desirable first to remove the particulates from the gasstream. Such an operation may be achieved by using an aqueous scrubbingmedium which is recycled within contact chamber 300 to the nozzles 318and 320 and is saturated with respect to sulfur dioxide, so that removalof sulfur dioxide in the contact chamber 300 cannot be effected.Following such contact, phase separation is effected to remove allparticulates contained in the scrubbing liquor, prior to recycle withinthe chamber 300.

Alternatively, a basic solution may be fed to the nozzles 318, 320,which has the effect of removal of larger quantities of SO₂ in thechamber 300, so that lesser quantities are required to be removed in theduct 312. If longer residence times are required, a second contactchamber may be used and thereby enhance SO₂ removal.

It may be desirable under some circumstances to employ an entrainmentseparator between the contact chamber 300 and the nozzle 310 to assistin maintaining the specific conditions conducive to each stage.

Any particulate material remaining in the gas stream following thecontact chamber 300 and entering the scrubber section at nozzle 310 isremoved from the gas stream along with the SO₂. Thickening or filtrationof the resulting scrubbing liquor separates out the solids. For the casewhere a water-soluble scrubbing agent is used, for example, sodiumhydroxide or sodium sulfite, the filtered solution may be contacted witha hydrated lime slurry in a conventional dual alkali process with thebasic sodium sulfite being returned to the nozzle 310.

An alternative arrangement is shown in FIG. 9, in which the inlet pipe321 is in a vertically-downward orientation, with the nozzle 318 againlocated in the entrance to the chamber 310', where space constraints donot permit the normal horizontal flow. An optional additional nozzle 324may be provided on the downstream side of the baffle 316 for additionalscrubbing, as required, both in chamber 300 and 310'.

In a further alternative arrangement, the dual-fluid spray nozzle 318,in the embodiment of FIG. 8 or 9, may be located to spray the aqueouscontact medium countercurrent to the direction of flow of the gas streamentering the chamber 300. For example, the nozzle 318 may be locatedvertically axially over the lower closure to the contact chamber 310 tospray contact medium vertically upwardly towards the gas inlet port,parallel to the baffle plate 316. In this arrangement, the spray nozzle320 may be located axially in the upper closure to the chamber 310 onthe gas discharge side of the baffle 316 to spray contact mediumvertically downwardly, countercurrent to the gas flow direction.

Referring to FIG. 10, there is illustrated therein an embodiment ofapparatus 400 for removal of solute gas from a gas stream which isrelatively particulate-free and which does not employ a dropletcoalescing fan, such as is employed in the embodiment of FIG. 1.

In particular, the sulfur dioxide-containing gaseous emissions from acoal-fired power plant tend to be relatively free from particulates,since the gas stream conventionally is passed through a high efficiencyelectrostatic precipitator, and is particularly suited to treatment bythe apparatus 400.

Apparatus 400, which takes the form of an in-duct scrubber, comprises aduct 410 which is divided into three individual chambers 412, 414 and416 by mist eliminators 418, 420 and 421, which serve to coalesce andremove liquid droplets from the gas stream passing from chamber 412 tochamber 414 and from chamber 414 to chamber 416, respectively. In eachof the chambers 412, 414 and 416 is situated a dual-fluid spray nozzle424, which is arranged to form sprays which are countercurrent to thedirection of flow of the gas stream 426, in each of the chambers 412,414 and 416.

The downstream end of the chamber 416 is connected through the misteliminator 421 via a duct 428 to a high efficiency mist eliminator 430,before the solute-gas free and liquid droplet-free gas stream isdischarged by line 432. Any convenient fan mechanism may be used toprovide the necessary pressure difference to carry the gas streamthrough the in-duct scrubber 400 to the exit from the mist eliminator430, such as an induced-draft (I.D.) fan 434 located at the downstreamend of the entrainment chamber 430.

Scrubbing liquor for the solute gas passes countercurrent to gas flowthrough the duct 410. Fresh, or regenerated, scrubbing liquor is fed byline 436 to the spray nozzles 424 in the chamber 416 to contact the gasstream passing from the upstream chamber 414 and remove any residualsulfur dioxide or other solute gas remaining in the gas stream. Thesolute gas-containing liquid droplets are coalesced and the resultingcoalesced liquid, containing dissolved solute gas is fed to a scrubbingliquor storage vessel 444.

Coalesced liquid from the last section in the demister 430 is passed byline 450 to the first liquid storage vessel 442, while coalesced liquidfrom the chamber 416 and from the other sections in the demister 430pass by lines 452, 454 and 456 respectively to the second liquid storagevessel 444. Liquor collected in the first vessel 442 is recycled by line458 to the demister 430.

Liquor collected in the second vessel 442 is forwarded by line 460 tothe spray nozzle 424 in the chamber 414 to contact the gas streampassing from the upstream chamber 412 and to remove further quantitiesof sulfur dioxide, or other solute gas from the gas stream. The solutegas-containing liquid droplets in the gas stream are coalesced in thedemister 420 and coalesced liquid is forwarded by line 462 to a thirdliquid storage vessel 446.

The liquid in the third storage vessel 446 is forwarded by line 462 tothe spray nozzle 424 in the chamber 412 to contact the gas stream 426entering the duct 410 and to remove sulfur dioxide or other solute gasfrom the gas stream. The solute gas-containing liquid droplets arecoalesced in demister 418 and the resulting coalesced liquid containingdissolved solute gas is fed by line 464 to a fourth liquid storagevessel 448. Pregnant scrubbing liquor is removed from the fourth vessel448 by line 466 for regeneration or discard, as appropriate, dependingon the nature of the scrubbing liquor.

In the duct 410, therefore, the solute gas-containing gas stream iscontacted countercurrently by the scrubbing liquor, so that the gasstream, itself containing decreasing concentrations of solute gas, iscontacted by a scrubbing liquor having a decreasing concentration ofdissolved solute gas in the direction of flow, and each contact stage isfollowed by a demisting operation to remove solute-containing liquiddroplets from the gas stream before the next contact step.

The in-duct contact apparatus 400 may be employed, as noted above, forremoval of sulfur dioxide from the tail gas from coal-fired boilers.Similarly, the apparatus may be used for the removal of chlorine dioxidefrom bleach plant emissions. The ability to employ simple misteliminators for liquid droplet removal and simple induced draft orforced draft fans leads to a substantial cost savings in comparison tothe embodiment of FIG. 1, where the fan is required to perform turbulentmixing and agglomeration functions associated with the presence ofsignificant quantities of particulates.

The apparatus 400 also may be adapted to employ different reagents inthe separate stages of scrubbing, with each stage effectively beingisolated from the next stage by the demisters 418 and 420. Similarly,rather than the countercurrent flow of the scrubbing liquor with respectto gas flow, the same scrubbing liquor may be fed in parallel to each ofthe several stages of scrubbing, again with each stage of scrubbingbeing isolated from the next stage by the demisters 418 and 420. Thescrubbing liquor may be regenerated, as required, prior to beingrecycled to the absorption equipment.

EXAMPLES

The following specific Examples illustrate the use of the gas reactingapparatus of the present invention for the purpose of removing SO₂ fromsynthetic gases by NaOH and NH₃ aqueous solutions and a lime slurrycontaining MgO.

EXAMPLE I

This Example illustrates the use of the gas reacting apparatus of FIG. 1for the purpose of removing SO₂ from a synthetic gas stream containingabout 1100 ppm SO₂, 21%V O₂ and the balance nitrogen, by absorption intoaqueous NaOH solution of sufficient concentration of active sodiumalkalinity.

In this type of removal, absorption accompanied by chemical reactiontakes place between the SO₂ and NaOH to form soluble sodium-basedsulfite, bisulfite and sulfate compounds, which effectively traps SO₂ inthe solution. With the caustic system having an initial active sodiumconcentration of 0.3M (pH 12.4), a liquid- to-gas ratio of 1.0 USG per1000 acf of gas treated and a ratio of active molar concentration ofsodium to moles of SO₂ inlet of 1.2:1, 99% SO₂ removal was effected. Toeffect the same degree of SO₂ removal but at a lower pH of 6.2, a L/Gratio of 4.75:1 was required.

When the concentration of the aqueous NaOH solution was increased to0.5M active Na (pH 12.5), a liquid-to-gas ratio as low as 0.5 USG per1000 acf of gas treated was required to operate the reactor to effect99% SO₂ removal. These results were obtained by using three spraynozzles in series to form three separate reacting zones within a singleconduit. The nozzles were oriented countercurrently to the gas streamflow. The pressure drop across the conduit was less than 2 inches W.G.during these tests.

EXAMPLE II

This Example further illustrates the use of the gas reacting apparatusof FIG. 1 for the removal of SOz from synthetic gas stream containingabout 1100 ppm SO₂, 21% V O₂ and the balance nitrogen, by scrubbing withan ammoniacal solution.

The SO₂ removal efficiency averaged well above 95% which was maintainedat this level as long as the NH₃ -to-SO₂ feed stoichiometry was higherthan 1.9:1. With an NH₃ -to-SO₂ feed stoichiometry of from 1.8 to 2.0:1,the effect of liquid flow rate on SO₂ removal over the range of liquidrates of 0.005 to 0.5 USGM (corresponding to a L/G ratio range from 0.17to 1.15) was observed to be minor and in general high removalefficiencies were obtained ranging from 95 to 99%.

In these tests, the ammonia gas feed to the system was introduced withthe atomizing gas (air) into three pneumatic, dual-fluid nozzlescoaxially disposed in series in a conduit and using recycled scrubbingliquor as the liquid phase. It was observed that the high turbulence,swirling and pressure conditions prevailing at the nozzles enhancedsubstantially the chemisorption of the sulfur dioxide in the sprayedliquid phase.

In this method of ammonia injection, there was also evidence ofsubstantial suppression of a plume (commonly associated with ammoniascrubbing operations) exiting the apparatus in all of the tests soconducted and it may have been due to the manner in which the gaseousammonia was admitted to the system.

The Table below shows the SO₂ removal efficiency obtained as a functionof the reactor outlet pH and NH₃ /SO₂ stoichiometry employed at an L/Gof about 1.0 USG per 1000 acf of gas treated.

                  TABLE                                                           ______________________________________                                        NH.sub.3 /SO.sub.2                                                            Stoichiometry                                                                              Reactor pH                                                                              SO.sub.2 Removal Efficiency                            ______________________________________                                        1.09         3.9       55                                                     1.24         5.4       81                                                     1.52         6.6       92                                                     1.89         7.4       95                                                     2.00         8.5       99                                                     ______________________________________                                    

EXAMPLE III

This Example further illustrates the use of the gas reacting apparatusof FIG. 1 for removal of SO₂ from a synthetic gas stream using a limeslurry containing MgO. In this case a dolime assaying 35.9 wt % Ca and20 wt % Mg in the form of a finely divided powder, was slaked to give aslurry of some 1.9 wt % solids loading.

A synthetic gas stream containing about 1200 to 1400 ppm SO₂ and 21% Voxygen was produced at the rate of 550 to 650 acfm by adding SO₂ gasfrom cylinders to the inlet air stream. The temperature of the gas wasambient.

The system was operated in a recirculating mode during which continuousaddition of make up dolime slurry was added at the rate of 0.32 lb/minfor 145 minutes to provide for the required stoichiometric amount ofalkalinity and to maintain the recycled tank pH at a prescribed level ofbetween 6.0 and 7.0. Under such pH conditions, it was found that a highconcentration of dissolved alkalinity (present as magnesium sulfite) inthe reacting liquor occurred, resulting not only in a well-bufferedreacting solution but also in a scale-free operation of highreliability.

SO₂ removal of 95 to 97% was achieved with the gas reacting apparatus atgas-to-liquid ratio of 4.5 gal/10³ acf. This scrubbing efficiencyremained close to the above values for the duration of the test.

Operating experience with the gas reacting apparatus of FIG. 1 usingdifferent commercially available reacting agents has shown that, formost of the systems studied, under optimal pH conditions and reagentconcentration, an L/G of only 1 to 5 USG per 1000 acf of gas treatedappears to be adequate to maintain uniformly and consistently high SO₂removal. For reactive systems, such as the sodium and ammonia-basedsystems, the apparatus provides excellent SO₂ gas removal in excess of99% and, if necessary, also efficient simultaneous removal ofparticulate matter in excess of 99.6%, and yet permitting aliquid-to-gas ratio in the range from 0.17 to 0.5 USG per 1000 acf ofgas treated.

This low L/G ratio requirement employed by the gas reacting apparatus ofthe invention should not only reduce both capital and operating costs toa fraction of the costs related to traditional removal devices, butshould also enable easy integration into flue gas ductwork of existingoil or coal-fired boilers due to its compact size. As shown particularlyin FIG. 6, the apparatus of the invention can be configured to easilymeet site requirements.

SUMMARY OF DISCLOSURE

In summary of this disclosure, the present invention relates to thegas/liquid, gas/liquid/solid and gas/gas/liquid mass transfer art andmore particularly to an improved method and gas reacting apparatus forwet mass transferring of solute gases from process gas streams into aliquid or slurry reacting medium, wherein the mass transfer operationmay be a purely physical phenomenon or may involve solution of thematerial in the absorbing liquid or slurry, followed by reaction withone or more constituents in the absorbing liquid or slurry medium.

The improvement provides an apparatus in which accelerated absorptionand reaction of solute gases can be effected as a result of the largeinterfacial surface area for mass transfer, plurality of reaction zones,intimate contact, increased residence time and turbulent mixingprevailing therein.

While an improved apparatus and method have been described in detail,various modifications, alterations and changes may be made withoutdeparting from the spirit and scope of the present invention as definedby the appended claims.

What I claim is:
 1. A method for wet mass transferring at least onesolute gas from a process gas stream into a reacting medium for saidsolute gas, comprising:(a) providing an elongate conduit having an inletthereto and an outlet therefrom and which is divided into a plurality ofindividual gas-atomized liquid contact zones; (b) passing a gas streamcontaining at least one solute gas comprising sulfur dioxide into theinlet end of the elongate conduit; (c) injecting a liquid reactingmedium comprising an aqueous alkaline medium capable of adsorbing saidat least one solute gas directly into said gas stream under an atomizingpressure of about 20 to about 100 psi from a plurality of dual-fluidspray nozzles coaxially disposed in said conduit one in each of theindividual gas-atomized liquid contact zones so as to form a spraypattern from each of said nozzles filling homogeneously thecross-section of said conduit in each of the individual gas-atomizedliquid contact zones and containing liquid droplets ranging in size fromabout 5 to about 100 microns, thereby to form a plurality of individualcontact spray zones whereby mass transfer of said at least one solutegas into said reacting medium is carried out in a very efficient way dueto the large interfacial surface area for mass transfer, turbulentmixing and relatively long residence time generated therein; (d)contacting the gas stream exiting each individual gas-atomized liquidcontact zone with demisting means to agglomerate and remove entraineddroplets from the gas stream before the gas stream passes to the nextsuch zone and out of the outlet end; and (e) discharging a clean gasstream from the outlet end.
 2. The method of claim 1 wherein said gasstream is contacted counter-currently with a spray pattern from thedual-fluid spray nozzle located in each of said contact zones.
 3. Themethod of claim 2 wherein said liquid reacting medium is passed inparallel to each of said contact zones.
 4. The method of claim 2 whereinsaid liquid reacting medium is passed countercurrently to the directionof flow of said gas stream through said conduit to successive ones ofsaid contact zones.
 5. The method of claim 4 wherein said conduit hasthree said contact zones.
 6. The method of claim 1 wherein said reactingmedium is a sodium-based solution sprayed at a rate of from 0.1 to 5 USGper 1000 acf of gas treated.
 7. The method of claim 1 wherein saidreacting medium is an ammonia-based solution sprayed at a rate of from0.1 to 3 USG per 1000 acf of gas treated.
 8. The method of claim 1wherein said reacting medium is an aqueous alkaline slurry solution. 9.The method of claim 1 wherein said reacting medium is a metal oxidebased slurry solution.
 10. The method of claim 1 wherein said gas streamcontains solute gases and particulate matter.
 11. The method of claim 10wherein simultaneous removal of both solute gases and particulate matterare effected.
 12. The method of claim 11 wherein said gas streamcontains sulfur dioxide and fly ash.
 13. The method of claim 12 whereinremoval of both sulfur dioxide and fly ash are effected to a degree offrom 95 to 99.5%.
 14. The method of claim 1 wherein said atomizing gaspressure is about 25 to about 55 psi.
 15. The method of claim 1 whereinthe atomizing gas is air.
 16. The method of claim 1 wherein said spraydroplets have a size ranging from about 5 to about 30 microns.
 17. Themethod of claim 1 utilized to effect removal of SO₂ from the off-gasesemanating from a coal-fired boiler by employing an aqueous lime slurryas said reacting medium to remove fly ash, unreacted lime and reactedlime plus residual SO₂ simultaneously to a substantial degree.
 18. Themethod of claim 1, wherein said gas stream contains solute gas andparticulate matter and, prior to said first reacting loop system, thegas stream is passed through a contact chamber wherein the gas stream iscontacted with an aqueous contact medium which removes at leastparticulate matter from said gas stream.
 19. The method of claim 18wherein said aqueous contact medium comprises an aqueous mediumsaturated with respect to said solute gases, whereby only particulatematerial is removed from said gas stream in said contact chamber. 20.The method of claim 18 wherein said gas stream passes from an inlet inan upper portion of said contact chamber first in a vertically-downwardflow path in said contact chamber and then in a vertically-upward flowpath in said contact chamber to an outlet in said upper portion of saidcontact chamber, and said gas stream is contacted by said aqueouscontact medium both in said vertically-downward flow path and in saidvertically-upward flow path.
 21. The method of claim 20, wherein saidaqueous contact medium is injected using a dual fluid spray nozzlelocated to form a spray pattern of fine liquid droplets varying in sizefrom about 5 to about 100 microns in both the downwardly-moving flowpath and the upwardly-moving flow path.
 22. The method of claim 21,wherein said spray pattern of aqueous contact medium is co-current tothe direction of flow of said gas stream in said vertically-downwardflow path and countercurrent to the direction of flow of said gas streamin said vertically-upward flow path.
 23. The method of claim 21, whereinsaid spray pattern of aqueous contact medium is countercurrent to thedirection of flow of said gas stream in said vertically-downward flowpath and countercurrent to the direction of flow of said gas stream insaid vertically-upward flow path.
 24. The method of claim 1 wherein saidreacting medium is one in which said at least one solute gas is absorbedwithout any accompanying chemical reaction.
 25. The method of claim 1wherein said reacting medium is one in which said at least one solutegas is absorbed and is chemically converted by the reacting medium.